As the number of users of wireless communication systems, such as cellular radio systems, is on the increase, and as rapid data transmission in these systems is becoming more and more common, it is essentially important to increase the capacity of the system by improving the performance of the system. One solution to this problem is to use one or more adaptive antenna arrays instead of sector antennas. In an antenna array, separate antenna elements are typically positioned close to each other, i.e. at about half a wavelength from each other. Typically, there are a sufficient number of antennas in such arrays to achieve the desired coverage area.
When adaptive antenna arrays are used, the basic idea is that narrow radiation beams are used which are directed as directly as possible at the desired receiver. Methods generally known in the use of adaptive antenna arrays can be divided into two main groups: radiation beams are directed at the receiver, or the most suitable beam is selected from several alternative beams. A suitable beam is selected for downlink transmission, or the beam is turned on the basis of the information received from the uplink. The reuse of frequencies can be made more efficient and the power of the transmitter can be reduced, because, owing to the directivity of the antenna beams, interference with other users diminish.
The directing of antenna beams is implemented in the uplink in a digital system by dividing the signal in baseband parts into I and Q branches and by multiplying in a complex manner (phase and amplitude) the signal of each antenna element with appropriate weighting coefficients, and subsequently by summing up the output signals of all antenna elements. The adaptive antenna array comprises in this case not only antennas but also a signal processor, which automatically adapts antenna beams by means of a control algorithm by turning antenna beams to the direction of the strongest signal measured.
The directivity of the beams can also be implemented by analogically generating with fixed phasing (Butler matrix) orthogonal radiation beams, in which the phase increases antenna by antenna. In the method, it is simply measured which beam receives most signal energy, in other words in which beam the signal is strongest, and this beam is then selected for the transmission.
In cellular radio systems, the network part of the radio system, such as a base station, typically transmits one base-station-specific uninterrupted signal or several for example sector-specific uninterrupted signals, called pilot signals in the CDMA system. The subscriber terminal listens to the pilot signal in crossover situations, for example, so as to be able to set up a connection to the base station and thus to the cellular radio network. The pilot signal is a non-data-modulated signal, transmitted with a known spreading code and on the same frequency band as the actual traffic channels. The pilot signal can be distinguished from traffic channels only on the basis of the spreading code. The base station of each system transmits a pilot signal of its own, on the basis of which the subscriber terminals can distinguish the transmissions of different base stations from each other.
The problem is to generate an uninterrupted signal covering the whole antenna sector, because due to the effect of multipath propagation, for example, the signals entering the receiver, when being of opposite phases, cancel each other, and thus a fade maximum is brought about.
U.S. Pat. No. 5,577,265 discloses a system by means of which, using adaptive antenna arrays, the effect of fades can be averaged out, and an uninterrupted pilot signal covering the whole antenna sector can be achieved more reliably. The system comprises an antenna array, which comprises two antennas positioned at a distance shorter than the wavelength of a carrier wave from each other. The phase of the input signal of both antennas is changed, and thus a phase difference is achieved for the signals to be transmitted. Hereby, the occurrence of fades is more random and the fade maximums last a shorter time, but the solution disclosed in the publication cannot eliminate the fades completely.
Another known solution for minimizing the problems caused by fades and for generating an uninterrupted pilot signal covering the whole antenna sector is disclosed in publication Ericsson Review No. 3, 1999. The solution disclosed in the publication is based on the use of separate sector antennas as a part of an adaptive antenna array: the antenna solution comprises not only an array antenna element but also an added radiation element. However, the problem with this solution is that the number of antennas must be increased.